Just as there are reports, photos, and even video footage (not of the dropping/falling, mind you) of people with bumpers (the Apple specific bumper) and other types of cases on their iPhone 4's and the front glass shatters - even one guy that stated his daughter dropped his fully encased (except the front display) iPhone 4 maybe 1-2' onto a carpeted floor and when he picked it up, he was pretty shocked to see the "spider-web" of cracks because it shattered.
It ain't how it falls, it's how it hits... and while the bumper will absorb some of the impact, because of it's tight fit and design it's still going to transfer a lot of that kinetic energy into the frame of the phone which then transfers it to the front glass and back, so, you could still find yourself with a busted phone even so. 🙁